Extruded sections or tubes of light alloy are usually made from round ingots, which have been made by continuous casting and must be subjected to a heat treatment that is known as homogenization and serves to eliminate heterogeneities of the as-cast structure of the ingot. The most widely used alloys are usually heat-treated at temperatures between about 560.degree. and 580.degree. C. The ingots which have been heated to that homogenizing temperature must be held at that temperature for the time which is required for an adequate diffusion. For that purpose the ingots may be heated in a continuous furnace. Compared with a heating of a stack of ingots in a batch-type furnace, the heating of the ingots in a continuous furnace affords the advantage that all ingots are subjected to the same heat treatment. But in that case the walking beam conveyor must comprise ingot holders in a number which is adequate for the desired throughput rate so that each ingot will not only be heated to the homogenizing temperature but will be kept at this temperature for a predetermined time, which usually amounts to several hours. With the number of ingot holders, the length of the furnace will increase as does the space which is required for the furnace so that the ratio of furnace throughput rate to furnace volume becomes undesirable. Because the beams of the walking beam conveyor are subjected mainly to bending stresses by the ingots lying thereon, these beams must be properly supported. The supports for the walking beams extend through the furnace housing in openings which must permit the supports to move in the direction of conveyance to the extent of the length of a step of the walking beam conveyor. Even though these openings may be sealed with heat-insulating material, heat losses through the openings are inevitable and may be substantial because the furnace has a substantial length.
In an attempt to reduce the length of such continuous furnaces, the heating chamber to be traversed by the ingots can be divided in the direction of conveyance into two heated compartments, i.e. a heating-up compartment and a holding compartment, in which the ingots are held at the temperature to which they have been heated. In such a divided heating chamber the ingots can be heated to the homogenizing temperature by gases at a temperature in excess of that homogenizing temperature so that the heating-up time and the length of the heating-up compartment can be substantially reduced. Nevertheless, such continuous furnaces require considerable space. For this reason it has been suggested to preheat the ingots to the required homogenizing temperature in separate continuous furnaces although this involves a substantially higher capital and structural expenditure.
The heat-treated ingots must be reheated so that they will have an adequate ductility before they can be extruded. The required reheating temperatures are about 50.degree. to 100.degree. C. below the homogenizing temperature. The reheating of ingots to be extruded is usually performed in separate furnaces, in which additional energy is consumed. It appears to be desirable to homogenize the ingots immediately before they are extruded so that an additional reheating to a temperature at which the ingots can be extruded will no longer be required. But a combination of a homogenizing treatment and a reheating to an extrusion temperature in a common continuous furnace is usually unpractical because the furnace would require a large space and usually there are space limitations in existing extrusion plants.
From German Pat. No. 752,031 it is known that light alloy ingots can be conveyed through a heat-treating furnace by means of a revolving vertical chain conveyor having conveying beams which are provided with members for carrying the light alloy ingots. Compared with intermittent conveyors, such chain conveyors have the substantial disadvantage that the speed of travel must necessarily be the same throughout the path of travel and during the transition from the rising course to the descending course of the chain the ingots must be lifted from the carrying members adjacent to the upper reversing sprockets and must subsequently be caught by the carrying members. Besides, in the known furnaces provided with such vertical chain conveyors all ingots contained in the heating chamber can be subjected only to a uniform temperature so that the heating chamber cannot be divided into a heating-up compartment and a holding compartment.